Method of growing a crystal or crystalline layer by means of a direct current arc discharge



z v -lal J. R. DRABBLE ET AL G A CRYSTAL May 6, 1969 3,442,719 METHOD OF GROWIN OR CRYSTALLINE LAYER BY MEANS OF A DIRECT CURRENT ARC DISCHARGE Filed May 24, 1968 3,442 719 METHOD OF GROWING A CRYSTAL OR CRYSTAL- LINE LAYER BY MEANS OF A DIRECT CUR- RENT ARC DISCHARGE ,lohn R. Drabble, Exeter, Devon, and Anthony William U.S. Cl. 148-1.6 17 Claims ABSTRACT OF THE DISCLOSURE A method of growing a crystal of a given material is described wherein the crystal forms part of one electrode of a given polarity of a direct current are discharge, while the other electrode of opposite polarity is formed at least in part of the said material and is consumable to transfer the material to the first-mentioned electrode.

This invention is concerned with a method of growing a crystal and is a continuation-in-part of our application Ser. No. 492,728, filed Oct. 4, 1965,

According to the present invention, in a method of growing a crystal of a given material including said element, said crystal forms at least part of a first electrode of a given polarity of a direct current arc discharge which is established between said first electrode and a consumable second electrode having a polarity opposite to that of said first electrode and being formed at least in part of the said material.

The term are discharge in the present specification should be understood to include both continuous and intermittent discharges, and spark discharges; and the term crystal should be understood to include crystals and crystalline layers.

The current through the arc discharge is preferably varied to control the rate of deposition of material on the first electrode.

The said first electrode is preferably provided initially with a horizontal surface on which the crystal is deposited, said horizontal surface being disposed vertically beneath or vertically above the said second electrode.

In one preferred method according to the invention, the second electrode is formed wholly or in part of said material and the arc discharge is established in a gaseous atmosphere the composition of which is controlled whereby combination of said element with a constituent of said atmosphere takes place and a crystal of a predetermined compound including said element is grown at the first electrode.

The rate of growth of the crystal may be increased I automatically as growth thereof proceeds.

The first electrode may be formed wholly of the element or compound to be'deposited. Alternatively, the first electrode may be formed of a material which is a seed for the said crystal.

The crystallisation of the material may be carried out in an atmosphere the partial pressure of the constituents of which may be controlled.

In one preferred embodiment, said first electrode is a negative electrode, although more usually it is a positive electrode. When the first electrode is connected as a negative electrode, the material may be (or contain) cerium oxide or lanthanum oxide.

nited States Patent 3,442,719 Patented May 6, 1969 The invention also includes a crystal grown by any of the methods herein described and claimed.

The invention will be described, by way of example only, with reference to the accompanying drawings, in which:

FIGURE 1 shows schematically an. apparatus by means of which, using the method according to the invention, single crystals of nickel oxide (melting point 2050 C. approximately) may be grown.

FIGURE 2 shows a modification of part of the ap paratus for growing crystals by the method of the present invention.

Referring to FIGURE 1 of the drawing, a positive electrode 10 and a negative electrode 11 together constitute a direct current arc discharge apparatus (the rest of which is not shown), the positive electrode 10 being disposed vertically beneath the negative electrode 11. The negative electrode 11 is consumable and is in this example formed by a nickel oxide rod, produced by sintering nickel oxide powder.

When an arc 12 is struck between the electrodes 10, 11, nickel oxide is deposited from the electrode 11 on a horizontal surface 13 provided on the positive electrode 10. In the example illustrated the positive electrode 10 is formed of the material which is to be deposited, namely nickel oxide, but the electrode 10' may alternatively be formed of conventional electrode material, such as carbon, capped with a material which may or may not interact with the material to be deposited and which may, in addition, be a seed for the crystallisation of said material.

The arc 12 is established in the conventional manner If, however, the electrical conductivity of the electrodes 10 and 11 is not sufficiently high at normal temperatures to provide a sufficiently electrically conductive path, it is necessary to provide such a path before establishing the arc. For nickel oxide, and other materials whose electrical conductivity increases with increasing temperature, the temperature of the electrodes 10, 11 may be raised by external heating means (not shown) until their electrical conductivity is sufficient to permit an arc to be struck.

One such external heating means which has been used successfully employes a hollow cylindrical heater which surrounds the electrodes 10 and 11 and which comprises nickel chrome wire woilind on a silica-former which is maintained at a temperature of approximately 1000 C.

Alternatively, or in addition, an electrically conductive path may be provided by means other than heating. For example, a metallic path for the current has been provided by reducing the surface of the electrodes 10, 11 to nickel.

As the nickel oxide is deposited on the surface 13 of the positive electrode 10 it crystallises, and in time a crystal 14 of nickel oxide grows on the positive electrode 10. In the example illustrated the electrodes 10, 11 are 1 cm. in diameter and an arc gap of 0.5 cm. is used. With a current of 12 amps a nickel oxide crystal 14 of 1.5 cm. diameter can be grown in air at atmospheric pressure at a rate of 1.5 cm. per hour. Growth can be continued in this way until the crystal 14 has attained any desired length.

The diameter of the crystal 14 can be controlled within wide limits by varying the current passing through the arc 12. For example, with the arrangement of the example illustrated, a current of 3 amps gives a diameter of crystal 14 of about 4 mm. and a current of 20 amps gives a diameter of crystal 14 of about 2 cm.

The continued presence of the are 12 maintains a zone 15 of liquid nickel oxide above the crystal 14 being formed. crystallisation of the nickel oxide takes place gradually behind the liquid zone 15, and the heating effect of the current passing through the crystal 14 maintains crystal 14 at a high temperature. By maintaining the whole of the crystal 14 at a high temperature in this way, thermally induced strain in the crystal 14, which may otherwise arise as a result of the different rates of cooling at different parts' of the-crystal 14, is relieved.

At the completion of growth, the rate of cooling of the nickel oxide crystal 14 can be controlled according to the invention, since it is dependent on the current passed through the crystal 1.4.

The rate of deposition of material (nickel oxide) can be controlled according to the invention within fine limits, since it is dependent on, inter alia, the current passed through the are 12, affording, therefore, precise control of crystal growth.

The rate of growth is also dependent on the electrode geometry. It has been found advantageous to use a relatively small diameter negative electrode 11 when small crystal growth rates are required and to increase the diameter when it is required to increase the growth rate. For growing single crystals it is usual to begin with a small growth rate and to increase the growth rate gradu= ally as growth proceeds. This increase can be effected advantageously by, for example, providing a negative electrode 11 of tapering cross-section (see FIGURE 2), de creasing towards the positive electrode 10, whereby the effective cross-section of the electrode 11 increases as the crystal growth proceeds, while at the same time the current through the are 12 is increased at a suitable rate.

The composition and properties of the resulting crystal 14 can be affected by introducing predetermined amounts of impurity material in the negative electrode, enabling a. doped crystal 14 to be grown. In the example illus trated, the room temperature value of the electrical con ductivity of the nickel oxide crystal 14 can be controlled by the addition of a suitable amount of lithium oxide to the negative electrode 11.

The electrodes 10, 11 are mounted (by means not shown, in the interests of clarity) for fine relative movement, so that the stability of the are 12. may be controlled. For example, the voltage difference across the are 12, which varies with the arc gap for a particular current, may be used to operate a servomechanism (not shown) which controls the gap between the electrodes 10, 11 in such a way as to maintain this voltage at a constant value to better than one part in five hundred. Thus, by using a constant current supply, such a servomechanism provides automaticelectrode feed under constant arc gap conditions.

Although the apparatus illustrated is designed for the production of single crystals 14, the method according to the invention is not confined to this application. Th s the method may also be used for coating a body with a crystalline layer of material. For such an application the body to be coated would be used as the positive electrode 10, the position of the body with respect to the negative electrode 11 being adjusted, and the time of exposure to the are 12 being controlled, until a crystalline layer of the desired thickness is formed. The exposure time of the" body to be coated to the are 12 could, for example, be controlled by moving the body continuously relative to the are 12, the rate of movement determining the exposure time.

The atmosphere in which the arc 12 is struck may comprise a gas other than air, and the pressure of such gas may be other than atmospheric. The partial pressures of the constituents of the medium in which the are 12 is struck may, furthermore, be controlled. The nature of the crystal 14 formed may in some cases be influenced by these factors and by the nature of the materials constituting the electrodes 10, 11. Thus it is possible, by the method of the present invention, to grow crystals of compounds formed by combination of an element in the negative elec-= trode 11 with a constituent of the atmosphere in which the are 12 is struck. In this way a crystalline oxide may be de limi ed on he PDSiti t electrode by the inclusion of oxygen in the said atmosphere, while a nitride deposit may be formed when the arc 12 is struck in an atmosphere of nitrogen. A nickel oxide crystal has, for example, been grown using electrodes 10, 11 of pure nickel, the are 12 being struck in air at atmospheric pressure.

Examples of other crystalline materials which have been grown using the present invention are as follows:

(a) Iron oxide (melting point 1,565 C. approximately) using electrodes of pure iron in air at atmospheric pressure, for example, with a current of 5 amps, producing a crystal with a diameter of 7 mm. approximately.

(b) Cobalt oxide (melting point l,935 C.) using electrodes of sintered cobalt oxide in air at atmospheric pres-- sure, for example, with a current of 6 amps, producing a crystal with a diameter of 7 mm. approximately.

(c) Titanium oxide (melting point 1,800 C. approximately) using electrodes of sintered titanium oxide in an atmosphere of argon, for example, with a current of 5 amps, producing a crystal of diameter approximately 5 mm.

(d) Vanadium oxide (melting point 1,920 C. approximately) using electrodes of sintered vanadium oxide in an atmosphere of argon, for example with a current of 8.5 amps, producing a crystal of diameter approximately 6 mm.

(e) Nickel ferrite (melting point 1,600 C. approximately) using electrodes of sintered nickel ferrite in air at atmospheric pressure, for example with a current of 12 amps, producing a crystal with a diameter of approxi mately 1 cm.

It will be seen that the method according to the invention is particularly suitable for the growth of crystals of refractory materials, that is to say, materials having melting points in excess of 900 C. In addition to the examples referred to above, the following other materials could possibly be grown by this method: aluminium oxide; barium titanate; calcium tungstate; chromium oxide; cobalt ferrite; copper; germanium; magnesium oxide; manganese oxide; silicon; silicon carbide.

It has been found that certain materials may be grown by deposition through an arc discharge onto the negative electrode, the positive electrode being formed wholly or in part of the said material. Examples of such materials include cerium oxide and lanthanum oxide.

Referring to FIGURE 3, there is shown diagrammati cally an arrangement by means of which e.g. cerium oxide crystals may be grown by the method of the present vention.

A positive electrode 10' and a negative electrode 11 are connected across a direct current source (not shown) and a direct curernt arc discharge 12 is established between said electrodes. The negative electrode 11 is dis posed vertically beneath the positive electrode 10', the positive electrode 10 being consumable and formed wholly or in part of cerium oxide.

When the direct current are discharge 12 is established between the electrodes .10, 11, cerium oxide is deposited through the discharge in crystalline form on a horizontal surface 13' provided on the negative electrode 11'. As the process continues, a crystal 14 grows on the negative electrode 11, the positive electrode 10 being eroded.

The negative electrode 11 is conveniently also formed of the material to be deposited, in this case cerium oxide. Alternatively, the negative electrode 11 may be formed of conventional electrode material, such as carbon, capped with a suitable seed material for the crystallisation of the crystalline material (in this case cerium oxide) on the surface 13'.

Indications are that crystals of relatively better quality but of smaller size, may be grown by a modification of the apparatus described above with reference to FIG- URE 3 in which the negative electrode 11 is disposed vertically above the positive electrode 10'.

It will, however, be appreciated that, while the invention has been illustrated with reference to the formation Of Y S 9f refractory material, and particularly suited for this application, it is not restricted to the formation of such crystals but may be generally applicable to other material which can be used as one of the electrodes in a direct current are discharge,

The fact that, in the method according to the invention, the crystalline material can, in many cases be maintained at a high temperature (in the region of its melting point) during growth is advantageous as it is favourable to accurate and uniform doping, that is, controlled distribution of selected impurities in the material,

The fact that the method of the invention does not depend for its success on mechanical devices renders the associatedapparatus relatively free of complication and of the difiiculties associated with mechanical vibration and its effect on crystal growth. Also, since the method of crystalgrowth according to the invention is, essentially, electrically rather than mechanically controlled, it is readily susceptible to automation methods.

We claim: i

.1 A method of growing a single crystal of a given material comprising establishing a direct current are dis charge between a consumable first electrode of either positive or negative polarity and a second electrode of opposite polarity, said single crystal forming part of said second electrode, said first electrode being formed at least in part of said material, consuming at least a portion of said first electrode in operation to directly transfer material to be deposited onto the second electrode for subsequent crystallizaiton thereon as asingletcriystal, and said are discharge being the sole means of maintaining a zone of liquid material on the crystal which is being formed,

2. A method as in claim 1 wherein the rate of growth of the crystal is increased automatically as growth thereof proceeds, by providing a first electrode with a tapering cross section, decreasing towards the second electrode, whereby the efiective cross section of the first electrode increases as growth of the crystal proceeds, while at the same time the current through the arc is increased at a suitable rate,

3, A method as in claim 1 wherein said first electrode is a positive electrode,

4, A method as claimed in claim 1 wherein the cur rent through said are discharge is varied to control the rate of deposition of material on said second electrode,

5., A method as claimed in claim 4 wherein the said second electrode is provided initially with a horizontal surface on which the crystal is deposited, said horizontal surface and said first electrode being disposed vertically relative to each other,

6. A method as claimed in claim 1 wherein the said first electrode is formed at least in part of an element and the arc discharge is established in a gaseous atmosphere the composition of which is controlled whereby combination of said element with a constituent of said atmosphere takes place and a crystal of a predetermined compound including said element is grown at the said first electrode,

7. A method as claimed in claim 6 wherein said atmos phere contains oxygen and an oxide of the said element is grown as acrystal at the said second electrode,

8,. A method as claimed in claim 1 wherein the said second electrode is formed wholly of the material to be deposited.

9. A method as claimed in claim 8 wherein the said second electrode is formed of a material which interacts chemically with the material to be deposited 10. A method as claimed in claim 8 wherein the said second electrode is formed of a material which does not interact chemically with the material to be deposited,

11. A method as claimed in claim 1 wherein the said first electrode contains a predetermined proportion of a material with which it is desired to dope the resulting crystal.

12. A method as claimed in claim 1 wherein the crys tallisation of the material is carried out in an atmosphere the partial pressure of the constituents of which is controlled.

13. A method as claimed in claim 1 wherein the material of the crystal is a refractory material, that is to say, material having a melting point of at least 900 C.

14. A method as claimed in claim 13 wherein said refractory material is nickel oxide;

.15. A method as claimed in claim 1 wherein said first electrode is afnegative electrode.

16. A method as claimed in claim 15 wherein said ma terial is cerium oxide.

17. A method as claimed in claim 15 wherein said ma' terial is lanthanum oxide References Cited UNITED STATES PATENTS 2,965,456 12/1960 Clark 23-273 2,970,895 2/1961 Clark m- 23-273 3,232,745 2/1966 Rummez et a1. u, 148-15 XR 3,234,051 2/1966 Kiifer et al -5, 23-301 XR 3,314,769 4/1967 Rudness et a1, a a. 23 -301 3,325,392 ,6/1967 Rummez 148- l XR L, DEWAYi IE RUTLEDGE, Primary Examiner, P. wEINsTEiN, Assistant Examiner,

US, Cl, X.R., 

